A coupling device according to the preamble of claim 1 is previously known from GB 2293221 A. This coupling device comprises a male coupling part, a corresponding female coupling part and an intermediate metal seal. The metal seal is preloaded by urging said coupling parts against each other. The male coupling part is provided with two projections which fit into helical grooves in the female coupling part, and the coupling parts are urged towards each other by a rotation of the male coupling part in relation to the female coupling part. In the coupling device according to GB 2293221 A, a valve of the type indicated in the preamble of claim 1 is arranged in the female coupling part. The spring member urging the valve body towards its advanced position, in which position the valve body closes the internal bore of the female coupling part, is arranged inside the internal bore in contact with the fluid therein. Consequently, the valve body will be subjected to the force from the spring member as well as the forces from the fluid pressure inside the internal bore. If the female coupling part is disconnected from the male coupling part there is a risk that the external pressure from the surrounding medium will cause an inwardly directed force on the valve body that exceeds the combined forces from the spring member and the fluid pressure inside the internal bore, in which case the valve body will be unable to close the internal bore and prevent an undesired exchange of fluids between the surroundings and the internal bore. This could for instance ensue when the coupling device is installed in a subsea environment at a great depth.
In this description and the subsequent claim, the term “fluid” refers to a flowing medium in gaseous or liquid form. Consequently, the expression “fluid-tight seal” implies a seal that is liquid-tight as well as gas-tight.